I agree with Mohan. Even if the bug is not reproducible, it does exist in the system. Right? If in case in future this kind of defect is raised by customer then QA will have to give explanation as to why QA missed it. So at that time you can show the filed bug.
Hence I feel that you should file the bug right away.
Yes, you should file it and keep it in watch state(Milestone should not be stated there)
Bug should contain all the configuration details and probable steps.
Even if you don't remember the exact steps, file it with proper screen shots, logs etc.
Later, if anyone again get such bug, any tester can see the record in the bug description or can provide more information there.
It depends on the company's procedures!!!
In some companies they log such issues as tracking items, So that the QA/Dev assigned can do further research on the specific item!!
I would also say...
Raise it :Mention that its Not always reproducible, and Describe exactly how you got it....
These cases should be logged as bugs and share the information to the devloper team, with maximum info to reproduce it,, as there are many chances that the developer can check his code( and see if there is any error in his code )and fix it...
At times it's difficult to observe or replicate the bug which was found mins ago..due to slow App/DB server response/patch deployment activities/Corrupt data etc thereby keeping some of the defects uncovered.
In such cases even screenshots would not help much..A better option is to use Video Capture facility using tools like SnagIT etc for tester's
record. According to me, tester should report this bug with low severity, since it could be replicable on simulating the test with the exactly the same inputs and under same enviornmental conditions..In case at later stage it couldn't be replicated with all possible combinations of similar i/p paremeters even on multiple trials, then it can be discarded or left at developer's descretion.
You can go ahead n log the bug... but u need to repro it for developers in order to fix it.. before logging it try to find what exactly should be done to repro it.... if you cant find it... then mention it saying it rarely repro's in the bug report...